Genetic Types and Charging Patterns of Tight Gas in the Upper Paleozoic Formation of Northeastern Ordos Basin, China

Abstract

Tight gas has become globally recognized for their huge resource potential. The northeastern Ordos Basin in central China has become an exploration target in recent years due to the great potential of tight gas resources in the Upper Paleozoic Strata. In this study, by analyzing the geochemical characteristics of the source rocks and the natural gas, the genesis of the Upper Paleozoic tight gas was identified. In addition, the accumulation process was discussed by combining the studies of hydrocarbon generation and burial-thermal history. Results show: Three sets of source rocks, Carboniferous Benxi (C₂b), Permian Taiyuan (P₁t) and Shanxi (P₁s) formations, consisting of mudstone, carbonaceous mudstone and coal, were developed in the study area. The source rocks contain abundant type II and type III organic matters, and have entered mature to high-mature stage, while source rocks in the southeastern part of the study area have entered overmature stage due to the activity of Zijinshan magmatic rock. The tight gas has high content of methane (average 93.99 %). The methane carbon isotope (δ¹³C₁) value (average −37.05‰), the ethane carbon isotope (δ¹³C₂) value (average −26.4‰), and the compositions of light hydrocarbons reflect that the tight gas is organic origined, thermogenic and mostly terrigenous sourced gas. The evolution of the transitional to continental depositional environment of the source rocks result in the variation of the geochemical characteristic of the Upper Paleozoic natural gas in the study area. The analyses of fluid inclusions indicate a long-period continuous charging process in the study area from 165 Ma to 65 Ma. The sand bodies closed to or interbedded with the source rocks make the natural gas charging from source rocks to reservoirs in short distance with high efficiency, while the faults and fractures provide paths for the natural gas migrating to reservoirs far from the source rocks vertically. The huge gas generation potential of the source rocks and continuous distribution pattern of sand bodies provides favorable conditions for the development of the large-scale tight gas reservoir.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018